1. Field of the Invention
The present invention relates to a limited slip differential for a four-wheel drive vehicle.
2. Description of the Related Art
In recent years, electronically controlled on-demand type four-wheel drive vehicles based on FF (front engine, front-wheel drive) vehicles have widely been put into practical use. As is described in Japanese Patent Publication No. 2005-162097A, an electronically controlled on-demand type four-wheel drive vehicle is configured such that power of an engine is delivered to front wheels via a transmission, and part of the power is electronically controlled to be outputted to rear wheels via a transfer clutch and hence has superior performances with respect to driving stability and suppression of slips. In addition, the characteristics of the four-wheel drive vehicle can be increased further by providing in a differential thereof a locking mechanism or a distribution controller for limiting a differential action between its two output shafts to prevent wheel spin.
In the case of the electronically controlled on-demand type four-wheel drive vehicle based on the FF vehicle, however, a cylinder block of the engine, a power steering mechanism for steering drive wheels, a front cross member which is provided in the front of the vehicle as a body, an exhaust pipe and the like are mounted in a front part of the vehicle, and a transmission and a transfer case are assembled onto the vehicle by making use of spaces eventually left between them. Consequently, although a distribution controller for the front wheels is attempted to be incorporated in the transmission or the transfer case, enlargement of a housing of the transmission or the transfer case due to the incorporation of the distribution controller immediately results in interference of the housing with the cylinder block or the other various components accommodated in the front part of the vehicle, whereby easy equipment of a suitable distribution controller has been prevented.
In the event that a mechanical slip limiting mechanism such as a viscous coupling for limiting the differential action is used for the distribution controller, although the resulting distribution controller becomes compact in size and hence the equipment of such a distribution controller is facilitated, the manner of limiting the differential action becomes simple, and it becomes difficult to satisfy demands with respect to the performance of the vehicle on various running conditions. In the event that the differential action limiting characteristics are made more active, although the traction performance can be increased, the difference in rotational speed between the two output shafts or wheels cannot be accommodated sufficiently in the differential, and hence, a braking phenomenon or the like is generated when the vehicle corners, and on the other hand, in the event that the differential action limiting characteristics are made less active, sufficient countermeasure may not be able to be obtained in case there is caused a difference in rotational speed between the two wheels.
To cope with this, although, for example, a mechanism made up of a multi-plate clutch including an electromagnetic coil is preferable as the distribution controller in that the differential action limiting characteristics can be set arbitrarily, the mechanism of such a unit becomes complex, and in association with this, a large installation space becomes necessary, thus a limited slip differential unit equipped with such a distribution controller having been unable to be easily used on the FF-based four-wheel drive vehicles which have little extra space in the interior of the engine compartment.